System for burning low volatile fuels



April 18, 1961 P. H. KOCH ET AL 2,980,084

SYSTEM FOR BURNING LOW VOLATILE .FUELS Filed March 18, 1958 4 Sheets-Sheet l ATTEMPERATOR 174 REHEATER 12a INVENTORS Paul H. Koch BY Judson J. Fink ATTORNEY Apnl 18, 1961 P. H. KOCH ETAL 2,980,084

SYSTEM FOR BURNING LOW VOLATILE FUELS Filed March 18, 1958 4 Sheets-Sheet 2 FIG. 2

1N VEN TORS Paul H. Koch BY Judson J. Fink AT TORNEY April 18, 1961 P. H. KOCH ETAL 2,980,084

SYSTEM FOR BURNING LOW VOLATILE FUELS Filed March 18, 1958 4 Sheets-Sheet 3 FIG.3

Paul H. Koch By Judson J. Fink ATTORNEY April 18, 1961 P. H. KOCH ETAL SYSTEM FOR BURNING LOW VOLATILE FUELS 4 Sheets-Sheet 4 Filed March 18, 1958 INVENTORS Paul H. Koch ,Judson J. Fink ATT ORNEY SYSTEM FOR BURNING LOW VOLATILE FUELS Paul H. Koch, East Orange, and Judson J. Fink, Union, N.J., assignors to The Bab'cock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Mar. 18, 1958, Ser. No. 722,322

3 Claims. (Cl. 122--480) The present invention relates to a method of and apparatus for burning low volatile fuel, and more particularly to a system of combustion wherein some partially cooled gaseous products of combustion are used to preheat combustion air to relatively high temperatures to efficiently burn low volatile fuels while other further cooled gaseous products of combustion are recirculated to the combustion zone of the fuel at a relatively low temperature to temper the gaseous products of fuel combustion. The invention is exemplified in a vapor generating and heating unit Where lowvolatile solid fuels are burned at high temperatures for efficient combustion and the gaseous products of the fuel combustion are tempered to reduce or eliminate the deposition of slag onthe heat exchange surfaces of the vapor generating and heating unit.

Heretofore, the combustion of low volatile solid fuel, such as char and coal, has proven to be difficult without high carbon losses and difficulty in depositiing slag on the surfaces of heatexchange elements associated with the generation and heating of the steam. We have found that low volatile coals and char can be successfully and efficiently burned by the use of highly preheated combustion air, such as in a cyclone furnace of the general type shown in US Patent 2,357,301. Such air should be heated to a temperature in excess of 600 F.; and preferably over 700 F. for good combustion of the fuel.

Frequently in high pressure, high capacity vapor generating and heating units utilizing cyclone furnaces it is desirable or necessary to use controlled quantities of'relatively cool recirculated gases to regulate the temperature of vapor heated in the unit, or to reduce the slagging effects of the fuel on the'vapor heating surfaces. The latter slagging effect is reduced or eliminated by the use of recirculated gases where the actual effect of such gases is to temper the heating gases either within or' closely adjacent the furnace of the unit so that any slag entrained in the heating gases. leaving the furnace will be in a dry state. Under such conditions, the recirculated gasesneed not be at a'temperature' at all comparable'with the combustion air temperaatures used in the unit, and, in fact, should be at least 100 F. lower in temperature. It is accordingly economically desirable to cool the gases. in the economizer section of the'unit before the recirculated gases are extractedfrom theheating' gas flow path and reintroduced into the furnace portion of theunit.

In accordance with'our present invention, we provide a plurality of gas flow passages over the economizer sec tion of the unit, where the heat absorbing abilities ofthe several sections of economizer are different in the 'dilfer ent heating gas flow paths. Such an arrangementefiectively utilizes the heat in the heating -gases, and at the sametime provides adequate gas temperatures for heating air in an air heater positioned downstream, ina gas flow sense, of the. economizer section. Such-an arrangement efliciently provides .the necessary air temperature and gas temperature desired for thefldifierentportions of the unit and furthermore'efiectsthis'lin a simple, compact and economical arrangement.

re States Pat n 34 above the downflowgas pass 36.

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The various features of novelty which characterize our invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For

a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which we have illustrated and described a preferred embodiment of the invention.

Of the drawings:

Fig. 1 is a side elevation, in section, of a high capacity steam generating and heating unit constructed in accord ance with the invention; 7

Fig. 2 is a horizontal section showing part of the apparatus'shown in Fig. 1 as taken on the line 2--2;

Fig. 3 is a plan, in section, of a portion of the vapor generating and heating unit taken on line 33 in Fig.-1;= and Fig. 4 is an elevation, in section, taken on line 4-4 of Fig. 3.

The illustrative steam generating and superheating. unit involves a furnace including fuel burningmeans in the form of one or more cyclone furnaces 10, a primary furnace chamber 12, a secondary furnace chamber 14, and an economizer including the section 16, and a superheaterincluding the sections 26, 28 and 30. The superheater' sections 26, 28 and 30, and the economizer are subject to the how of heating gases from the secondary furnace chamber 14, through a gas pass in the upper part 32 of which the gases flow horizontally to a gas turning space The cyclone fuel burning furnaces 10 of the illustrative steam generating and 'superheating unit are of the type shown and described in the patent to Bailey et al-.; 2,357,301. the cyclone furnace has walls including steam generating tubes connected into the circulation of the unit. Such connections involve, for example, the upper cyclone header 100 and the lower cyclone header 102 connected by semi-circular wall tubes. From the upper, header 100 risers 104 lead to the steam and water drum 106. From the water space of this drum one or more tubular downcomers 108 lead to the lower part of the unit. The lower ends ofthe downcomers are connected to the cyclone furnace lowerheader 102 by circulators 110. v

From the cyclone furnace the high temperature'heat ing gases flow through a throat 112 into the primary fur-j nace chamber ,12 and the gas flow continues thence. up-

wardly between thehorizontally spaced screen-platens 114 into the lower part of the secondary furnace cham ber14. i

' The lower end of the downcomer 108 may also'be connected'by suitable circulators 116 to lower sidef. wall headers such as 118 from which furnace side wall tubes Continuing sections .of these tubes form the cyclone furnace'throat '112 and thence along the front furnace wall 'to the drum 106.: Other tubes leading. from the header 128 have sections forming the screen platens 114' and the wall 132 dividing the primary furnace chamber 12 from the secondary furnace chamber 14. These tubes likewise continue along the'furnace wall 130 to openinto'; the drum 106. Stillother tubes 134. lead from thefdrum 7o or'header 128, and have succeeding sections .136, gong-T therear furnace wall,-6,4. Othersections 138"a1"1 d. 1i40 define the archv 142,.iwith inclined sections .144 at the base of the lateral'gas pass. The tubes continue Like the other components of the furnace;-

as upright screen sections 146 between which gases pass to the downflow gas pass 36. The tubes continue as furnace roof sections 148 leading to the drum 106.

, Steam flows from the steam space of the drum 106 through superheater steam supply tubes 150 to a header 152. From the header, two rows of tubes conduct steam to a header 158 positioned at a lower level in the down pass 36. The tubes 154 of one row extend along the left hand wall 156 (Fig. l) of the downflow gas pass, and a second row of tubes has upright sections disposed across the gas fiow at positions adjacent the tube sections 146, but spaced therefrom. Lower tube sections'160 continue along the right hand wall 162 of the downflow gas pass 36 to a position adjacentthe level of the header 158. At this levelthese tubes extend in the horizontally spaced sections 164 to the header 158.

From the header 153 the steam flows through the tubular sections 170 and thence through the banks of return bend tubes constituting the primary superheater section 30, from whence it flows to the primary superheater outlet header 172 and through the tubular connections 173 including spray attemperator 174, to the intermediate superheater header 176, which is the inlet of the secondary superheater 28. From this header the steam flows through the bank of return bend tube section 28, and through the U-tubes of the superheater section 26 to the secondary outlet header 178. From this header the steam flows to a point 'of use, for example, the inlet of a steam turbine (not shown).

From a stage of the steam turbine, lower pressure steam is exhausted to flow through tubular connections 180 to the reheater inlet header 182. From this header the steam flows through the return bend tubes constituting the reheater section 22. From these tubes the steam flow continues to the intermediate reheater header 184 and thence through the tubes constituting the reheater section 24. Reheated steam from these tub-es flows into the header 186 and thence through tubular connections 188 to the inlet of a succeeding stage of the steam turbine. In accordance with this invention, the heating gases leaving the superheater sections 30 are divided into separate flow paths for use in heating combustion air to relatively high temperatures, and for providing tempering gmes of relatively lower temperatures to the furnace 14. This is accomplished by the installation of partitions 40 and 41 in the lo-werportion of the gas pass 36. The partitions divide the gas flow path into separate sections and ext-end transverseiy across the gas pass and are attached at their lower ends to the wall portions forming the dust hoppers of the unit. Thus,"the' separategas passes are completely separated for the movement 'of partially spent heating gas through the lower portion of the gas pass 36. F As shown in Figs. 1, 3 and 4, three separate gas passes are formed, namely the passes 42, 44 and 45. The gas pass 42 is further divided to discharge the gases passing therethrough to'separate regenerative type air heaters 200. The duct connection between the gas pass 42 and each air heater includes the duct sections 94 and 96. The heating gases passing through the gas pass 42 thus are used directly to preheat combustion air in the air heaters 200, with the preheated combustion air thereafter delivered through the ductwork 202 to the cyclones 10.

The gas passages 44 and 45 are separately connected through ducts 47 to the inlet opening of separate fans 46 and are provided With suitable tightly closing dampers .56. The gases discharged from each of the fans 46 pass through a duct54 into the tempering gas distributing duct 53. As hereinbefore described, the tempering gases delivered to the duct 58 pass through the openings 60. into the furnace upstream of the radiant reheater elements 22.'

Since the tempering gasesare utilized to modify the temperature of the heating gases delivered to the radiant heating elements 22 and 24, maximum flow of tempering gases will occur at maximum fuel input to the cyclone furnaces. As the fuel burning rateisreduced in thjef a fuel input rating of from 60 to 70% of maximum input, depending upon the operating conditions of the particular installation. Thus, with tempering gas flow utilized as described, the fiowof gases through the gas passes 44 and 45 need not be proportional to the total flow of heating gases through the gas pass 36. To control the rate of tempering gas flow to the furnace 14, the duct 54 leading from the fans 46 is provided with flow control means, such as dampers 52.

With the heating gases passing over the superheater sections 30 being of a substantially uniform temperature, transversely of the gas flow path, the economizer sections 16, 16C and 16D, disposed respectively in the gas flow passageways 42, 44 and 45, will differ in their ability to absorb heat from the gases. In the illustrated embodiment of the invention, as shown particularly in Fig. 4, the economizer elements 16 positioned in the gas passageway 42 will be of considerably less depth than the correspondingly positioned economizer sections 16C and 16D positioned in the gas passageways 44 and 45. As

shown in Fig. 4, the economizer section 16 consists of four high elements, while the economizer sections 160 and 16D each consist of sixteen high elements.

In the described economizer arrangement, all of the economizer sections 16, 16C and 16D are connected for parallel flow of water from a common source such as the header 98. Furthermore, the heated boiler feed water from each of the separate economizer sections, is collected in a common manifold header for delivery to the drum 106. It is, of course, understood that the economizer sections 16, 16C and 16D can be connected for series flow of fluid therethrough or for any combination of series and parallel flow.

In an operating unit constructed in accordance with the principles of the invention, the economizer section 16 was constructed with six high elements, with the section containing 94 elements. The sections 16C and 16D were constructed with 20 high elements with each section containing 40 elements. When operated at designed unit output rating approximately 1,872,000 pounds of heating gases per hour passed over the economizer section 16 to the air heaters 200 to preheat the combustion air to approximately 700 F. At the same time approximately 560,000 pounds of heating gases passed over the economizer sections 16C and 16D to cool the gases recirculated to the furnace 14 to a temperature of about 500 F. For these conditions, the water flow rate through the economizer sections 16, and 16C and 16D was approximately 1,040, 000 and 280,000 pounds per hour, respectively, with the temperature of the water delivered to the header 100 being 502 and 493 F.. respectively.

Under the above described conditions, the high temperature preheated combustion air aided the combustion of low volatile coal, or char, so that satisfactory carbon loss'was attained. The recirculated gases delivered to the furnace 14 effectively eliminated slagging of ash on the surfaces of the radiant steam heating elements 22 and24. V

At lower loads on the unit,the amount of recirculated gases was reduced by adjustment of the dampers 52 until at approximately 65% of rated load, the flow of recirculated gases was stopped. Regulation of the dampers 50 and 52 may be accomplished by automatic or manual means. Under all conditions of operation, the construction and arrangement of the illustrated embodiment of the invention proved effective for the purposes described.

While in accordance with the provisions of the statutes wehave illustrated anddescribed herein thebest form and mode of operation of the invention now known to us, those skilled in the art will understand that changes may be made inthe form of the apparatus disclosed without departing from the, spirit of theinvention covered by our-claims, and that certain features of our invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. In a vapor generating and heating unit having a radiant heating zone and a convection heating zone con nected for serial flow of heating gases therethrough, vapor generating and heating elements positioned in said zones, means for producing said heating gases by fuel combustion, heat exchange elements including liquid heating elements positioned in said convection heating zone, bafile means for dividing the gas flow over said liquid heating elements into a plurality of separate parallel flow passages, a recirculated gas fan, means connecting said fan with a first only of said separate parallel gas flow passages and with said radiant heating zone for flow of recirculated gases in an upper range of fuel firing rates to temper the heating gases in said heating zone, means for regulating the flow of recirculated gases to said radiant heating zone, an air heater, means connecting the other of said gas flow passages only with said air heater for flow of the remainder of said gases to heat combustion air to a relatively high temperature, and means for discharging high temperature combustion air from said air heater to said fuel combustion means.

2. In a vapor generating and heating unit having a radiant heating zone and a convection heating zone connected for serial flow of heating gases therethrough, vapor generating and heating elements positioned in said zones, burner means for producing said heating gases by the combustion of low volatile fuel, economizer elements positioned in said convection heating zone, bafile means for dividing the gas flow over said economizer elements into a plurality of separate parallel flow passages, a recirculated gas fan, means connecting said fan with a first only of said separate gas flow passages and with said radiant heating zone to temper the heating gases in said heating zone, the economizer elements in said first separate gas flow passage having a greater gas cooling capacity than the elements in the other of said separate gas flow passages, means for regulating the flow of recir- 4 culated gases to said radiant heating zone, an air heater,

means connecting said other gas flow passage only with said air heater for flow of the remainder of said gases to heat combustion air to a relatively high temperature, and means for discharging high temperature combustion air from said air heater to said burner means.

3. In a vapor generating and heating unit having an elongated upfiow radiant heating zone and an elongated downflow convection heating zone connected for serial flow of heating gases therethrough, vapor generating and heating elements positioned in said zones, cyclone furnace means for producing said heating gases by the combustion of low volatile fuel, said furnace means discharging heating gases into the lower portion of said radiant heating zone, economizer elements positioned in said convection heating zone adjacent the lower end thereof, baffle means for dividing the gas flow over said economizer elements into a plurality of separate parallel flow passages, a recirculated gas fan, means connecting said fan with a first only of said separate gas flow passages and with said radiant heating zone to mix relatively cool gases with the heating gases in said heating zone, the economizer elements in said first separate gas flow passage having a greater gas-cooling capacity than the elements in the other of said separate gas flow passages, means for regulating the fiow of recirculated gases to said radiant heating zone, an air heater, means connecting said other gas flow passage only with said air heater for flow of' the remainder of said gases to heat combustion air to a relatively high temperature, and means for discharging high temperature combustion air from said air heater into said cyclone furnace.

References Cited in the file of this patent UNITED STATES PATENTS 

